Referring to FIGS. 1 to 3, a coupling unit 11 of a conventional protective case assembly 1 is shown to include an annular protrusion 14 extending outwardly from a peripheral edge of a back plate 12 of the protective case assembly 1, and an annular groove 15 formed in an inner peripheral surface of a casing body 13 of the protective case assembly 1 for receiving the annular protrusion 14. When the annular protrusion 14 is received in the annular groove 15, the back plate 12 and the casing body 13 of the protective case assembly 1 can cover three sides of a handheld device (not shown).
However, the coupling unit 11 of the conventional protective case assembly 1 uses a back side of the handheld device to push the back plate 12 so as to abut the annular protrusion 14 against a wall defining the annular groove 15. Thus, if the handheld device located in the protective case assembly 1 is slightly moved in a direction away from the back plate 12, the annular protrusion 14 can easily slide out of the annular groove 15, so that the back plate 12 is likely to detach and fall away from the casing body 13.